This disclosure relates to methods for producing transparent, fire resistant polycarbonate compositions and more particularly, to methods for producing transparent, fire resistant polycarbonate compositions comprising flame retardant salts.
Plastics, and in particular aromatic polycarbonates, are increasingly being used to replace metals in a wide variety of applications, from car exteriors to aircraft interiors. The use of plastics instead of metal decreases weight, improves sound dampening, and makes assembly of the device easier. Unfortunately, polycarbonates are inherently flammable, and thus require the addition of flame retardants. A variety of different flame retardant materials have been used, some of which are set forth in U.S. Pat. Nos. 4,028,297; 4,110,299; 4,130,530; 4,303,575; 4,335,038; 4,552,911; 4,916,194; 5,218,027; and 5,508,323. The challenge is to identify economical, environmentally friendly flame retardant materials that provide the requisite flame resistance, but without compromising desirable polycarbonate properties such as strength, color, and clarity.
There are three general processes known for the commercial manufacture of aromatic polycarbonates, which are illustrated in FIG. 1. The conventional interfacial process, as shown in FIG. 1A, and the phosgene-based melt process, as shown in FIG. 1B, start with the reaction of chlorine with carbon monoxide to produce phosgene. The third general process, a “no phosgene” melt process as shown in FIG. 1C, was developed to eliminate the use of phosgene in the process flow. Of these general methods, the “no phosgene” melt process shown in FIG. 1C is preferred since it prepares polycarbonates less expensively than the interfacial process and avoids the use of highly toxic phosgene.
Both types of melt processes (FIGS. 1B, and 1C) make use of a diarylcarbonate, such as diphenylcarbonate (DPC) as an intermediate, which is polymerized with a dihydric phenol such as bisphenol A (BPA) in the presence of an alkaline catalyst to form a polycarbonate in accordance with the general reaction scheme shown in FIG. 2. This polycarbonate may be extruded or otherwise processed, and may be combined with additives such as flame retardants, dyes, blowing agents, light stabilizers, fillers, reinforcing agents, heat stabilizers, antioxidants, plasticizers, antistatic agents, mold releasing agents, an additional resin, or combinations comprising at least one of the foregoing additives.
In the production of polycarbonates, several reactors are typically used in sequence to prepare the final product. The final reactors in this sequence subject the reaction mixture to both high temperature and high vacuum. This treatment assists in the removal of byproduct phenol, unreacted monomer and short oligomers, improving the overall quality of the final product. For products requiring flame resistance, flame retardant is typically added after the final target specifications of the polycarbonate composition have been met (e.g., molecular weight, % branching, etc.). The polycarbonate composition can then be palletized or may be fed as a polymer melt to a compounder where the additives are combined with the polycarbonate composition and extruded or injection molded into the desired product, e.g., sheet. There are many different types of flame retardants including the use of flame retardant inorganic salts. The flame retardant salts are typically added in solid form as milled or unmilled powders.
It is well known that compounding flame resistant salts in solid form with the polycarbonate composition can produce surface imperfections (i.e., inclusions) in the extruded product as well as impart haze. Some flame retardant salts have a melting temperature greater than the compounding and processing temperatures employed that can directly contribute to the severity and amount of inclusions. For example, potassium diphenylsulfon-3-sulfonate is a flame retardant salt as disclosed, for example, in U.S. Pat. No. 4,735,978, having a melting temperature greater than about 350° C. During the compounding and processing of the polycarbonate composition, the temperatures are typically maintained less than about 300° C., temperatures considerably less than the melting temperature of KSS. As a result, KSS functions similar to a filler material when added to the polycarbonate extrudate in the reactors. That is, particles of KSS are distributed throughout the polycarbonate composition in order to impart the desired flame resistance to the extruded product. As one would expect, the size of the flame retardant salt particles can affect the amount of haze produced in the extruded product. Milling the flame retardant salt into smaller particles can help reduce the level of haze. However, for applications requiring optical quality, the reduction in haze may not be sufficient for the desired application (to justify the expense of milling). Moreover, milling does not reduce the level of imperfections produced in the fire resistant polycarbonate by any significant amount.
Accordingly, there remains a need in the art for methods of producing polycarbonates that are not only highly flame resistant, but also transparent and do not produce surface imperfections.